AFM Cantilever with in Situ Renewable Mercury Microelectrode

Technical Note pubs.acs.org/ac

AFM Cantilever with in Situ Renewable Mercury Microelectrode Peter Schön,*,† Joel̈ Geerlings,‡ Niels Tas,‡ and Edin Sarajlic‡,§ †

Materials Science and Technology of Polymers, MESA+ Institute for Nanotechnology, University of Twente, Enschede, The Netherlands ‡ Transducers Science and Technology, MESA+ Institute for Nanotechnology, University of Twente, Enschede, The Netherlands § SmartTip B.V., Enschede, The Netherlands S Supporting Information *

ABSTRACT: We report here first results obtained on a novel, in situ renewable mercury microelectrode integrated into an atomic force microscopy (AFM) cantilever. Our approach is based on a fountain pen probe with appropriate dimensions enabling reversible filling with (nonwetting) mercury under changing the applied pressure at a connected mercury supply in a dedicated experimental setup. The fountain pen probe utilizes a special design with vertical pillars inside the channel to minimize mechanical perturbation. In proof of principle experiments, dropping and hanging mercury drop were observed as a function of the applied pressure at the external mercury supply. Electrical conductivity occurred only through the mercury after filling, and the empty fountain pen probe showed excellent electrical insulation. This was demonstrated by chronoamperometric measurements in the electrolyte and by mechanical and electrical contacting of an ITO substrate with a mercury-filled and empty probe in air. Finally, cyclic voltammetry and square wave voltammetry were done in a static mercury electrode fountain pen configuration, demonstrating the principle usability of the mercury probe for electrochemical studies. Our findings are of fundamental importance as they enable further integration of a renewable mercury electrode probe into an AFM setup, which is the subject of ongoing work.

T

For application in SECM, the so-called sphere-cap mercury microelectrodes on Pt disks introduced by Daniele and coworkers for SECM are of particular interest.8 In combination with anodic stripping voltammetry (ASV), they were used for a variety of electroanalytical applications, including the study localized corrosion processes with high spatial resolution and the detection of various heavy metals in water and metal dissolution during zinc corrosion.9 Mercury film electrodes on Pt were utilized to generate concentration maps of heavy metal ions generated during corrosion.10 Kranz and Taillefert et al. used gold amalgam microelectrodes for SECM imaging of Mn2+ production during rhodrochrosite dissolution (MnCO3), a fundamental biogeochemical process.11 The above-described mercury microelectrodes need ex-situ preparation by electrodeposition from Hg2+ salt solutions before each measurement. Furthermore, operation of a dropping mercury electrode is excluded. Our approach comprises an in situ renewable mercury electrode integrated into an AFM cantilever. It is based on a fountain pen probe, which is an extension of the “dip-pen”12 and nanoscale dispensing “NADIS” concepts.13 These approaches all have the aim to deliver liquid or solutes contained in the liquid to the contact point in an AFM. The fountain pen concept was originally developed around the same time in two different

he introduction of the mercury electrode by Heyrovsky provided the basis for a number of pivotal electrochemical techniques, including polarography and voltammetry.1 Since then, the mercury electrode has become an exceptional landmark in electroanalytical chemistry. Mercury electrodes provide outstanding electrochemical performance, both in terms of polarizability and sensitivity, due to the possibility of stripping analysis. The high overpotential of the mercury electrode allows for direct detection of analytes with negative reduction potentials, enabling corrosion studies of surfaces involving zinc and iron, for instance, which occur in the regime of the hydrogen evolution on platinum electrodes.2 In addition to the fundamental role in electroanalytical chemistry, mercury electrodes became pivotal tools in various fields, including hard and soft matter particle electrochemistry,3 biological membrane research,4 or the formation of metal−molecule junctions for applications in molecular electronics,5 due to the unique properties of mercury. In recent years, various mercury (ultra)microelectrodes have been reported, where preparation was by electrodeposition of mercury from Hg2+ salt solutions onto a variety of substrates including carbon fibers and various metal microdiscs.2b,6 The electrodeposited mercury adopts a thin-film or a sphericalsegment shape, depending on the experimental conditions during preparation. Microelectrodes provide higher electrochemical sensitivity due to smaller double-layer capacitance and lower ohmic losses in comparison to macroelectrodes. They are used in scanning electrochemical microscopy (SECM) to probe the local reactivity of surfaces.7 © 2013 American Chemical Society

Received: February 22, 2013 Accepted: August 31, 2013 Published: August 31, 2013 8937

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Technical Note

laboratories.14 It contains an integrated channel to transport fluid from the base of the AFM probe to the tip, across the flexible cantilever beam. Applications include patterned SAM deposition,14 nanoelectrochemical metal deposition, gold particle deposition,15 deposition and patterning of DNA, and electric field induced delivery of proteins.16 In combination with atomic force microscopy, the concept FluidFM has been introduced and applied for dye and nanoparticle injection into single cells,17 displacement of single cells,18 and single cell virus infection.18,19 It is important to mention here that various designs of AFM tip-integrated SECM solid metal electrodes (Au, Pt) have been reported, allowing the simultaneous detection of topography and current signals.20 Their use to image biological membranes or as AFM tip integrated amperometric sensors has been reported, for instance. In the work presented here, we have used a fountain pen probe based on an existing FluidFM probe that was optimized for the large pressure required for the pumping of mercury through a small microchannel for an entirely novel application, namely an AFM probe with a renewable mercury microelectrode. The probe design included internal vertical pillars for mechanical stabilization. In addition to the internal pillars, the fountain pen probe channel height and the wall thickness were increased, to lower the liquid mercury pressure in the setup and to further mechanically stabilize the probe for working pressures of ∼6 bar.

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EXPERIMENTAL SECTION Microfabrication Process of the Fountain Pen Probe. The fluidic probes were fabricated in a wafer-scale process that combines standard silicon surface micromachining and bulk glass processing; the fabrication sequence is illustrated in Figure 1. The process starts on a standard (100) silicon wafer. On the selected wafer, a silicon nitride layer is deposited by low pressure chemical vapor deposition (LPCVD). The layer is patterned by reactive ion etching to form a small opening that will later serve as a probe aperture. After formation of the aperture, a sacrificial LPCVD polysilicon layer is deposited and patterned by RIE to form the layout of the microchannel. Next, another LPCVD silicon nitride layer is deposited to completely encapsulate the patterned sacrificial polysilicon layer. After the encapsulation, the probe layout and a relatively large inlet of the prospective microchannel are formed by selective RIE etching of silicon nitride. The silicon wafer is then anodically bonded to a glass wafer, in which through holes are powder-blasted. After the bonding, the probe cantilever is released by selective silicon etching in TMAH. At the same time, the sacrificial polysilicon layer is removed from the entire microchannel by etching through the inlet hole and the aperture. A small part of the silicon wafer on the backside of the inlet hole is preserved to serve as a mechanical reinforcement. Experimental Setup. The fountain pen probes were glued with UV glue to a specially designed polycarbonate block. By means of a fluidic Lee connector, mercury was fed into the block and probe. Electrical contact to the mercury was provided by a platinum wire inside the polycarbonate block. The polycarbonate block with probe was connected to a linear translation motor, which could vary the height of the probe with respect to the substrate with a resolution of 1 μm. The probes were mounted under a slight angle. The whole setup was mounted on top of an inverted microscope (Leica DMI 5000 M). A transparent indium tin oxide (ITO)-coated glass

Figure 1. Fabrication sequence: (a) deposition of a silicon nitride layer and etching of the prospective probe aperture; (b) LPCVD deposition and patterning of a sacrificial polysilicon layer to from the microchannel layout; (c) deposition of another silicon nitride layer and etching of the probe layout; (d) anodic bonding of a glass wafer with a powder-blasted inlet reservoir; (e) selective etching of silicon to simultaneously release the probe cantilever and to empty the microchannel.

plate of 100 μm was used for the conductivity experiments. The resistance between the center of the ITO substrate and the connecting wire was 14−26 Ω. The electrolyte solution in which the electrochemical experiments were conducted was contained by a rubber ring placed on the glass substrate. During the experiments, a syringe mounted in a translation table was used to apply pressure to the mercury inside the tubing. The pressure inside the tubing was monitored with a pressure sensor (Sunx DP-102). A pressure of ∼3 bar needed to fill a flat rectangular channel can be estimated based on the Young Laplace equation by 2γ cos(θ)/h. Here, θ ≅ 137 + −8° is the contact angle of the mercury to silicon nitride, γ = 487 mN/m is the surface tension of mercury, and h ≅ 3 μm is the channel height. In fact, we observed higher pressures in our experiments of ∼6 bar needed to induce the mercury flow possibly due to the confining effect of the reinforcing pillars in the probe channel. The inverted microscope, linear translation motor, and translation table were covered with plastic foil to protect it from possible mercury spills. For safety reasons, the setup was placed under a fumehood. Electrochemical Measurements. A PalmSens hand-held potentiostat (PalmSens, Utrecht, The Netherlands) or an Autolab PGSTAT10 potentiostat (Ecochemie, Utrecht, The 8938

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holds the reservoir (Table 1). The total length of the microchannel between the fluidic reservoir and the aperture in the cantilever is therefore 1450 μm. The aperture at the cantilever end was lithographically determined with a diameter of either 4 or 8 μm. Inside the channel, vertical pillars were added. The main purpose of the vertical pillars is to increase the mechanical strength of the microchannel walls in order to be able to apply rather large pressures (∼6 bar) without damaging the microchannel and to avoid cantilever bending. The latter aspect is essential for the further integration into an AFM setup with laser beam detection. To enhance withstanding of the mercury pressure, the channel height and wall thickness were increased. Our experimental setup included an inverted optical microscope, which allowed detailed observation of the mercury filling and flowing process of the fountain pen probe with mercury, as function of the applied pressure (Figure 3).

Netherlands) were used for the electrochemical experiments, which were carried out in a three-electrode arrangement. The AFM cantilever integrated mercury electrode was used as the working electrode. The reference (Ag wire or Ag/AgCl) and counter (Pt wire) electrodes were immersed in the electrolyte solution, as illustrated in Figure 3. Electrochemical measurements were done in 0.1 M NaClO4 or 0.1 M KCl.

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RESULTS In the work presented here, we utilized a fountain pen probe of specific design and dimensions for the reversible filling, with mercury under working pressures of ∼6 bar. The principle usability of this device as dropping and static mercury-type electrode was tested in an electrolyte environment. First, we describe the design and setup of the fountain pen-based mercury electrode. Fountain Pen Probe. The fountain pen AFM probes used for the experiments are based on FluidFM probes17−19 optimized for the application under significantly higher liquid pressure. A schematic of the probe is shown in Figure 2. The

Figure 3. Schematic (not to scale) of the experimental setup with the fluidic and electric connections. (A) Syringe with mercury supply, (B) pressure sensor, (C) fluidic connector, (D) polycarbonate block29, (E) cantilever, (F) Ag wire, (G) Pt wire, (H) electrochemical potentiostat, and (I) inverted optical microscope.

In Figure 4, optical images of (A) an empty and (B) a mercury-filled fountain pen probe are shown. The mercury flows between the central pillars, forming a capillary of less than 5 μm in width. This filling behavior is due to the nonwetting properties of the channel and pillar sidewalls. A mercury drop can be seen with a size in the regime of the aperture. The huge drop is formed on the ITO surface due to cohesion and feeding via the dropping electrode. Importantly, upon filling and mercury flow any mechanical perturbation, for instance bending of the cantilever, could not be observed via optical microscopy, demonstrating the pivotal mechanical stability of our probe. In order to establish electrical contact of the fountain penbased mercury electrode, the probe was equipped with an electrical wire contact inside, which was immersed into the mercury filling and connected to a voltmeter and to an electrochemical potentiostat for the electrochemical experiments. To proof electrical conductivity of the mercury fountain pen probe, it was mechanically contacted to an ITO substrate. A high ohmic resistance was found, indicating good isolating properties of the hollow fountain pen probe. Upon filling with mercury, a drop in resistance to 133 Ω occurred as the mercury flew out of the opening and wetted the ITO surface under formation of an electrical contact. We conclude that electrical conductivity is maintained via the mercury inside the fountain pen probe, while the hollow fountain pen probe itself reveals excellent electrical insulation. The usability and performance of the mercury fountain pen probe as the electrochemical electrode was tested in 0.1 M NaClO4. A potential of 0.1 V was applied to the mercury probe versus a silver wire. As a counter electrode, a platinum wire was

Figure 2. (A) Schematic (not to scale) of the fountain pen probe (side view, cross section). (B) Optical microscopy image of a fountain pen probe with stabilizing pillars and aperture.

probe consists of a silicon nitride cantilever with an embedded microchannel and a support block for handling. The microchannel in the cantilever connects the aperture at the end of the cantilever with a fluidic reservoir inside the block. Typical dimensions of the probes are given in Table 1. Thicknesses of the nitride layers, compromising the cantilever, were 470 nm. The microchannel inside the probe consists of a part which is located in the cantilever (150 μm) and an (additional) larger part of 1300 μm located in the remainder of the probe that Table 1. Typical Dimensions of the (FluidFM) Fountain Pen Probes

length width height

cantilever (μm)

channel (μm)

150 36 3.9

1300 24 3.0 8939

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Figure 4. Optical microscopy of the fountain pen probe, (A) showing the mechanically stabilizing pillars inside the empty capillary and (B) demonstrating the filling of the fountain pen probe with mercury: (1) mercury drop at fountain pen probe aperture, (2) mercury capillary inside fountain pen probe, (3) huge mercury drop formed due to cohesion on the ITO substrate, fed by the dropping mercury from the fountain pen aperture.

Figure 5. Amperometric signal recorded at the dropping mercury fountain pen probe; 0.1 M NaClO4; 0.1 V vs Ag wire; E = 0.1 V.

Figure 6. Two cyclic voltammograms recorded in a static mercury electrode fountain pen configuration measured before and after an intermediate renewing step of the mercury; 10 mM Ru(NH3)6Cl3 in 0.1 M KCl recorded at 100 mVs−1 scan rate; reference electrode: Ag/AgCl.

used. The current at the mercury probe was recorded as a function of time and as a function of the pressure applied to the mercury. Initially, no current was observed (Figure 5), as

explained by the empty fountain pen probe. Increasing the pressure to ∼6 bar at the external mercury supply, a sudden current onset was recorded at a time of ∼10 s as a result of 8940

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Figure 7. Square-wave voltammograms recorded at the static mercury fountain pen probe in dependence of the Ru(NH3)6Cl3 concentration in solution: 1, 3, and 10 mM Ru(NH3)6Cl3 in 0.1 M KCl recorded at f = 10 Hz, Eampl = 10 mV, with Ag/AgCl as the reference electrode.

mercury filling and dropping into the electrolyte. The current increased but with decreasing rate until a maximum current of 0.15 μA was obtained, followed by a sharp drop to 0.1 μA and immediate current increase to a saturation value and repeated dropping, as observed at conventional macroscopic dropping mercury electrodes. As there was no electroactive species present, the observed current response was due to a capacitive charging of the electric double layer, the mercury drop growth related to its continuous dropping. Mercury drop lifetimes were in the regime of a few seconds to ∼10 s, while the currents observed under mercury dropping ranged between 0.05 μA (minimal value) to 0.16 μA [maximal value (exceptional: 0.21 μA)]. Continuous dropping of the mercury electrode was observed as recorded in the current response, complementing the observations made with an inverted optical microscope. Reducing the pressure again under the critical value effected an abrupt stop of mercury flow, accompanied with instantaneous drop of the current to zero. After ca. 1 min, the pressure at the mercury supply was raised again above the critical value, leading to the onset of mercury dropping again, as observed by the chronoamperometric response. This demonstrates in a proof of concept experiment the principle controllability of flow and stop of flow of the mercury through the fountain pen probe. Importantly, leakage currents cannot be observed. This is an absolute prerequisite to enable probing of local electrochemistry. However, the current response indicates instabilities during mercury flow. For instance, after mercury flow onset (Figure 5 at t = ∼200 s), initial drop lifetimes were significantly shorter and after a further ∼30 s, previously observed lifetimes were recovered. We are currently working on the stabilization of the dropping process, including the configuration of a sophisticated pumping system to fine control the mercury liquid pressure and to enable homogeneous wetting and flow inside the capillary. We could further achieve conditions resembling a static mercury probe configuration, after stopping the mercury dropping. In a static mercury electrode configuration, cyclic voltammetry (Figure 6) of 10 mM Ru(NH3)6Cl3 in 0.1 M KCl was done, demonstrating the principle usability of the mercury probe for electrochemical studies, as indicated by the reversible

reduction/oxidation of the Ru[II/III](NH3)63+/2+. For this measurement, the pressure at the mercury syringe was first increased manually until a dropping of the mercury could be detected via the optical microscope, followed by relaxation due to pressure release until the mercury dropping stopped. Subsequently, the first voltammogram was recorded. Next, the mercury pressure was reduced, further effecting a drop of the faradaic current to zero (data not shown). This was followed by a repeated refreshing of the mercury probe via pressure increase, inducing renewed mercury dropping and again relaxation. Next, another cyclic voltammogram was recorded. Upon renewing of the mercury electrode inbetween two subsequent measurements, the voltammogram showed a reproducible trace, indicating similar electrode areas in both voltammograms. To demonstrate the capability of the mercury probe for quantitative electroanalytical measurements, square wave voltammetry was done with increasing concentrations of Ru(NH3)6Cl3 in 0.1 M KCl, as the analyte (Figure 7). The obtained superimposed voltammograms show a corresponding increase of the current signal (at approximately −180 mV), in accordance with the increasing Ru(NH3)6Cl3 concentration. A subject of ongoing work is the further stabilization of the dropping and control of the static drop configuration. This includes integration of a sophisticated mercury pressure and dropping control under direct fluidic connection to the fountain pen probe.21

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CONCLUSION Here, we introduced a novel type of mercury electrode based on a fountain pen probe. To the best of our knowledge, an in situ renewable mercury electrode integrated into an AFM cantilever has not been reported in the literature. In proof of principle experiments, chronoamperometry, cyclic voltammetry, and square wave voltammetry measurements were done in electrolyte, testing the principle usability for electrochemical studies. Essentially, stabilization of the dropping process and its control are further needed. This includes the configuration of a sophisticated pumping system to fine control the mercury liquid pressure and to enable homogeneous wetting and flow 8941

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inside the capillary. Central focus of ongoing work is the combination of the cantilever-integrated mercury electrode with a commercial AFM instrument. This includes the manufacture of appropriate holders, allowing connection of the mercury feed system and the simultaneous operation of the laser deflection of the AFM system. In conclusion, our results enable us to further integrate the in situ renewable mercury electrode into the AFM setup, in particular to enable combined AFM electrochemical measurements to simultaneously probe forces and electrical/electrochemical signals. This might open novel avenues in areas where mechanics are coupled to electrochemical or electrical properties, for instance in biological membrane research. In principle, other liquid metals or liquid metal alloys could be utilized in this regard as well. Moreover, our probe might find a use to locally measure or map conductivities on surfaces or selfassembled monolayers for molecular electronics applications.

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ASSOCIATED CONTENT

S Supporting Information *

Additional information as noted in the text. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +31-53-4893170. Fax: +31 (0)53 489 3823. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Excellent technical support from Remco Sanders is gratefully acknowledged. P.S. acknowledges many fruitful and inspiring discussions with Vesna Svetličić and Christine Kranz and also acknowledges the COST Action TD1002: “European network on applications of Atomic Force Microscopy to NanoMedicine and Life Sciences” for providing fruitful discussions and collaborations. This work was financially supported by the MESA+ Institute for Nanotechnology of the University of Twente, the strategic research orientation “Enabling Technologies”, and by NWO/STW through a Vidi-grant, SmartTip (probe fabrication costs).

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REFERENCES

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